I play a game called crusader kings 2. In the game, one cannot declare war on other rulers without a proper casus beli.
Was it so in the medieval world? Could I as a ruler declare war on someone just cause I can or would I always need to specify some reason howsoever flimsy?
To add on to this. How many wars were actually "declared" back then? I was under the impression that most fighting was either raiding someone elses land or defending your own against raiders, and not necessarily the sort of wars we are used to in history books.
Although I guess this question also enters "what is a war?" territory.
It's late antiquity but as an example Attila the Hun, someone you would not think would care about such thing as an excuse for war, used the bethrotal with Honoria (Western roman emperor Valentinian's sister) plea for help as an excuse for war, Honoria sent him her ring in an attempt to escape an engagement with a Roman senator and Attila used it as an excuse to invade if Honoria was not given to him in marriage (he claimed Honoria sending her ring was actually a betrothal promise), of course a marriage in the imperial family would have given him legitimacy in Roman eyes and that was out of the question, so he invaded.